Performance is relative in these old British cars, particularily those with the "A" series motor. I would suggest maximizing what the engine can do with small changes first before big mods. Start with a "spot-on" tuneup, coupled with one of the ignition points replacement kits that uses a electronic ignitor - like the Pertronix and others. The improvement in starting, idle, acceleration, and at-speed cruise is very noticable, and worth every penny. You still won't be able to pass anyone going uphill, however. Many pros recommend advancing the standard timing setting 2 degrees or so to take advantage of today's better fuels. Do a compression check to see if all 4 cylinders are still within tolerance.
The Morris Minor was engineered to provide "adequate" performance for minimum fuel comsumption for a population traveling short distances in crowded English communities. Maybe transplanting a "B" series motor might make the Minor go faster. Surely someone has done it and it's on-line somewhere ...